using System; class Program { static void Main() { int[] nums = { 1, 2, 3, 5 }; int n = nums.Length + 1; // total numbers including the missing one int total = n * (n + 1) / 2; // sum of first n natural numbers int sum = 0; foreach (int num in nums) { sum += num; } int missingNumber = total - sum; Console.WriteLine("Missing number is: " + missingNumber); } }